The principles of operation and general design criteria for PIN diode variable optical attenuators (VOAs) realized from silicon-on-insulator rib waveguide structures are described. We present as a benchmark the performance of devices based on the established VOA produced by Bookham Technology Plc, and demonstrate 25dB attenuation at less than 70mW with novel recessed dopant geometries. Optical and electrical simulation results for new, smaller cross-section VOA structures based on rib waveguides utilizing a 2mm high guiding layer are detailed and discussed. Experimental results demonstrating the successful fabrication of these structures and the significant improvements in performance attained are presented. In particular we show that the attenuation efficiency can be 30% higher than that of the larger structure, and that modulation bandwidths may approach 10MHz.